John Whittingdale, former political secretary to Mrs Thatcher, recalled how in 1990 he worked on the then Conservative prime minister's conference speech and persuaded her to joke about the Lib Dems' bird of liberty symbol.

But the Tory frontbencher said he has never been convinced Mrs Thatcher understood the joke, although he added the comparison of his party's former coalition partners seems "even more fitting" now following their general election wipeout in May.

Speaking in 1990, Mrs Thatcher told Tory delegates: "I gather that during the last few days there have been some ill-natured jokes about their new symbol, a bird of some kind, adopted by the Liberal Democrats at Blackpool.

"Politics is a serious business and one should not lower the tone unduly.

"So I will say only this of the Liberal Democrat symbol and of the party it symbolises - this is an ex-parrot.

"It is not merely stunned, it has ceased to be, expired and gone to meet its maker.

"It is a parrot no more. It has run down the curtain and joined the choir invisible. This is a late parrot. And now for something completely different."

Mr Whittingdale, speaking in Manchester during his first conference speech since gaining his ministerial role, said of events in 1990: "I was trying to persuade Margaret Thatcher to include a passage comparing the symbol of the Liberal Democrats to Monty Python's dead parrot.

"As many of you know, I succeeded, although I was never sure that she entirely grasped why it was funny.